I don't know if this is intentional or not so forgive me if so, but if a block has up to 5 mussels placed on it, and the block is destroyed, only one mussel will drop. This also applies to if the mussels themselves are broken by a non player, such as a create machine or iron rod from quark. This makes an auto farm impossible.

Important to note I am playing on Alex's Caves version 1.1.5, so this issue might not exist further on.